This invention relates to a rotor blade for a turbo machine, and is particularly, but not exclusively, concerned with a rotor blade for an axial compressor of a turbine engine.
Axial compressors for turbine engines typically comprise at least one rotor having an array of rotor blades arranged circumferentially about a hub. The rotor blades have an aerofoil cross-section with a suction surface and a pressure surface. The rotor is disposed within a casing which defines an annular flow passage through the engine, across which the rotor blades extend. The casing is disposed radially outwardly of the tips of the rotor blades so that a clearance gap is provided between the tips of the rotor blades and the casing thereby allowing the rotor to rotate.
In operation, the rotor is rotated at high speed. Air drawn into the engine is turned by the rotor blades. As the air is turned the pressure acting on the pressure surface of the aerofoil increases and the pressure acting on the suction surface of the aerofoil decreases.
A problem associated with this arrangement is that the clearance gap between the rotor blade and the casing provides a flow path for air to leak from the pressure surface of the aerofoil to the suction surface. This leakage flow interacts with the main stream flow between the rotor blades in the region adjacent the aerofoil suction surface and typically rolls up into a vortex, known as a clearance vortex, which extends downstream and away from the suction surface.
Mixing of the clearance vortex with the main stream flow between the rotor blades reduces the aerodynamic efficiency of the of the blade row.
Furthermore, the clearance vortex has an upstream velocity component which counteracts the oncoming flow and so reduces the net downstream velocity of the main stream flow. In normal operation, the main flow is sufficient to counteract the upstream velocity component of the clearance vortex and entrain the clearance vortex downstream in the main flow. However, tip leakage is problematic when the compressor is throttled because throttling reduces the downstream velocity of the main flow. In addition, throttling increases the pressure difference (i.e. lift) between the pressure surface and the suction surface which increases the amount of leakage flow which strengthens the clearance vortex. The main flow is thus less able to entrain the clearance vortex and the vortex grows away from the suction surface towards the pressure surface of an adjacent blade. Eventually the casing end wall flow is blocked by the upstream flowing clearance vortex, significantly advancing the onset of stall. The surge margin of the rotor is thus significantly reduced.
Known methods for reducing tip leakage include using shrouded rotor blades; applying treatments to the tips of the rotor blades or the casing, such as slots to improve flow characteristics over the tip; and incorporation of swept, angled or profiled rotor blades.
These methods are known to incur significant penalties with respect to weight, complexity and/or reduced aerodynamic efficiency.